In many electronic applications, electrical signals exhibiting a very rapid transition in voltage or current are required. For example, an electrical signal in the form of a step function with a very rapid step transition is a common input signal applied to a device or system under test to allow the transfer function of the device under test to be determined from the corresponding output signal. Such testing of very high-frequency devices requires step-function signals of correspondingly fast transition time.
Another example of the use of an electrical signal exhibiting very rapid transition in voltage or current is in the art of electronic sampling. In an electronic sampling system, a very narrow electrical impulse signal is applied to a sampling gate to capture a very short time interval of some signal being measured. In order to effectively sample signals of relatively high frequency characteristics, a sampling pulse of comparatively short time duration is required. Also, a sampling pulse of relatively high amplitude is often required to effectively drive the sampling gate.
Generation of signals exhibiting relatively rapid transition times is common in the prior art. Common devices applied for generation of such signals are step-recovery diodes, tunnel diodes, avalanche transistors, gas switches such as thyratrons, spark gaps, and Josephson-junction devices among others. Although the transition times of such prior-art devices are relatively rapid, such transition times are too slow, with the exception of the Josephson-junction devices, in comparison to the high-frequency capability of typical devices in the art that could be evaluated or their signals sampled by means of a more rapid transition signal. For example, a linear rf amplifier exhibiting a maximum upper frequency capability of 25 GHz would require an input test signal exhibiting a transition time of about 10 ps or less to effectively evaluate the amplifier transfer function. Further, a sampling pulse exhibiting a pulse width on the order of 5 ps or less would be required to effectively sample the 25 GHz signal at the upper frequency capability of such an amplifier. Josephson-junction devices exhibit suitable transition times, but lack sufficient output signal level for effective use in such applications. The fastest transition times available from other prior-art devices are on the order of 20 ps or slower. Further, the specific devices capable of providing the fastest transition times are also limited to comparatively small signal amplitudes. Therefore, common devices of the prior art cannot provide both the rapid signal transition and high signal amplitude required.
The nonlinear transmission line according to the present invention provides for signals of both rapid transition and high signal amplitude by means of a novel continuous nonlinear transmission-line structure. The theory of operation of nonlinear transmission lines is well known in the art and described in, for instance, a paper entitled "The Theory of Radio Shock Waves in Nonlinear transmission Lines," by R. Khokhlov, Radio Engineering and Electronic Physics, Volume 6, Number 6, Jun., 1960, page 817. The use of nonlinear transmission lines for generation of signals exhibiting fast transition times is also well known in the prior art and is described in, for instance, a paper entitled "Parametric Amplification along Nonlinear Transmission Lines" by R. Landauer, Journal of Applied Physics, Volume 31, Number 3, Mar., 1960, page 479. Landauer teaches a generally conventional microstrip transmission line incorporating a ferroelectric crystal dielectric Under very specific conditions, such a dielectric exhibits nonlinear properties which in turn results in a nonlinear transmission line when such a material is applied as the dielectric of a transmission-line structure.
The nonlinear transmission line according to the present invention is significantly different from the nonlinear transmission line taught by Landauer. The present invention comprises a semiconductor device of novel construction and operation further comprising a continuous transmission line which exhibits a highly non-linear characteristic. For example, the nonlinear transmission line according to the present invention may be configured comprising a single novel semiconductor diode or a single novel semiconductor capacitor comprising an insulating layer deposited on a semiconductor substrate, for example a metal-oxide semiconductor (MOS) capacitor. Therefore, the nonlinear transmission line according to the present invention comprising a novel semiconductor device is significantly different from the nonlinear transmission line incorporating a ferroelectric crystal dielectric as taught by Landauer.
The nonlinear transmission line taught by Landauer suffers from the limitation that it must be operated very near the polarization transition temperature of the ferroelectric crystal dielectric. Maintaining such accurate temperature control complicates a nonlinear transmission-line system as taught by Landauer and increases the cost to manufacture and operate such a system. The nonlinear transmission line according to the present invention overcomes that limitation in a nonlinear transmission line by the use of a semiconductor device to provide a variable capacitance characteristic in a nonlinear transmission line. Although semiconductor characteristics are somewhat temperature dependent, such temperature dependence is much less severe than that of a typical ferroelectric crystal requiring operation at a very specific temperature such as the polarization transition temperature. Therefore, the nonlinear transmission line according to the present invention may be operated over a very wide temperature range without temperature control. For example, a nonlinear transmission line according to the present invention could be operated over a temperature range of -40.degree. C. to +125.degree. C. without the need for any type of temperature control. Further, since the properties of the semiconductor device comprising the nonlinear transmission line according to the present invention may be controlled accurately in manufacture, a nonlinear transmission line according to the present invention may be manufactured for optimum operation at a specific temperature. The elimination of the need for complex, costly temperature control and the capability of operation over a very wide temperature range in a nonlinear transmission line are significant improvements over the prior art provided by the nonlinear transmission line according to the present invention.
Another limitation in the nonlinear transmission line taught by Landauer is that the ferroelectric crystal must be selected to provide suitable nonlinear properties at a usable temperature. That severely limits the available selection of dielectric materials suitable for a nonlinear transmission line according to Landauer. Such restrictions result in detrimental compromise of such transmission line properties as rf loss and maximum operating frequency. The nonlinear transmission line according to the present invention overcomes those limitations in a nonlinear transmission line by allowing utilization of high-performance rf semiconductor materials, for example, Gallium-Arsenide (GaAs) and Silicon (Si), common in the art of microwave rf components. In a nonlinear transmission line according to the present invention, the various critical properties of the nonlinear transmission-line structure, for example, loss, maximum operating frequency, propagation velocity, characteristic impedance, nonlinear characteristics, and others may be optimized for specific applications. The capability to preserve desired performance and optimize critical parameters in a nonlinear transmission line are significant improvements over the prior art provided by the nonlinear transmission line according to the present invention.
An integrated semiconductor device comprising a linear transmission-line system is described in U.S. Pat. No. 3,778,643 entitled "A SOLID STATE VARIABLE DELAY LINE USING REVERSED-BIASED PN JUNCTIONS," to Jaffe. Jaffe teaches a substantially standard microstrip transmission-line structure as is common in the art. Such a microstrip transmission-line structure comprises a semiconductor slab to which an extended grounded conductive plane is deposited on one surface of such semiconductor slab and a strip conductor is deposited on the other surface thus forming a common microstrip transmission-line construction. The linear transmission-line system taught by Jaffe further comprises, as specifically required elements in such a linear transmission-line system, a semiconductor material further comprising homogeneous semiconductor diode structure, a biasing means to apply required direct-current bias, and a signal source limited to specific comparatively low-frequency operation to assure slow-wave mode operation. Whereas the linear transmission line system taught by Jaffe is specifically designed for operation as a linear transmission line as recited by Jaffe in column 2, line 50 of U.S. Pat. No. 3,778,643, and specifically for operation in a slow-wave mode, its constructions and materials are not consistent with operation as a nonlinear transmission line or at extremely high frequencies well above the slow-wave mode frequencies. More specifically, to assure linear operation in a transmission-line system taught by Jaffe, the transmission-line system is constructed of materials and structures to assure that there is substantially no variation in the transmission-line capacitance as a function of the applied signal propagating in the transmission line. In the transmission-line system taught by Jaffe, the capacitance of the linear transmission line is a function only of a direct-current bias.
The nonlinear transmission line according to the present invention is significantly different in both form and function from the linear transmission-line system taught by Jaffe. The purpose of the present invention is to provide in a transmission line highly nonlinear operation with arbitrary input signals and operation at extremely high frequencies. Specifically, in the transmission line according to the present invention, the capacitance of the nonlinear transmission-line structure at any point along the non-linear transmission-line structure is highly a function of the actual signal voltage at that point in the nonlinear transmission line. Such capacitance variation as a function of the propagating signal is significantly different from the linear transmission-line system taught by Jaffe in which the transmission-line capacitance is independent of the propagating signal. Also, the present invention comprises a semiconductor device of novel construction and operation further comprising a continuous nonlinear transmission line which provides different transmission-line characteristics at different positions along the length of such transmission line and additionally provides highly nonlinear operation. Such novel construction of the nonlinear transmission line according to the present invention provides operation with arbitrary input signals without the need for any external biasing means and further provides output signals of extremely high frequency characteristics. Therefore, the nonlinear transmission line according to the present invention including no external biasing means and further comprising a novel semiconductor device providing operating with arbitrary external input signals is significantly different and novel from the prior art taught by Jaffe.
A limitation in the linear transmission-line system taught by Jaffe in comparatively high-frequency operation is that the properties of the homogeneous diode structure comprising the transmission-line capacitance are constant along the line length thus providing a homogeneous transmission line. Therefore, in the linear transmission line taught by Jaffe, the properties of the transmission line are everywhere substantially the same along the line as would be required for linear operation. Accordingly, the linear transmission line taught by Jaffe cannot be optimized for optimum operation with comparatively high frequency signals or for propagation experiencing a maximum nonlinear effect. The nonlinear transmission line according to the present invention overcomes that limitation in the linear transmission-line system taught by Jaffe in the propagation of signals of comparatively high frequency by a novel variable structure providing different transmission-line characteristics at different positions along the length of the nonlinear transmission line. Such structure allows for optimization of performance and critical parameters in the nonlinear transmission line according to the present invention. The capability to optimize desired performance and to optimize critical parameters along the line length in a nonlinear transmission line are significant improvements over the prior art provided by the nonlinear transmission line according to the present invention.
Another limitation in the linear transmission line system taught by Jaffe is that it incorporates as a specifically included element of the linear transmission line system a continuous-wave signal source of comparatively low frequency. The requirement of such a specific and limited source as an integral element of the linear transmission line system taught by Jaffe severely limits the use of such system. The nonlinear transmission line according to the present invention overcomes that limitation in the linear transmission-line system taught by Jaffe by elimination of such an integral signal source and by providing means of coupling any arbitrary external signal to the input of the nonlinear transmission line according to the present invention. Therefore, the nonlinear transmission line according to the present invention may be applied with any arbitrary input signal. For example, a single pulse or any manner of continuous signal may be input to the nonlinear transmission line according to the present invention. Further, the nonlinear transmission line according to the present invention may be used as an integral element in an otherwise conventional signal transmission system. For example, a nonlinear transmission line according to the present invention may be applied with a first coaxial transmission line connected to its input terminal and a second coaxial transmission line connected to its output terminal. In such application the nonlinear transmission line according to the present invention would provide correction for the signal degrading effects of the two coaxial transmission lines connected input and output. The capability of operation with external and arbitrary input signals in a nonlinear transmission line are significant improvements over the prior art provided by the nonlinear transmission line according to the present invention.
Another limitation in the linear transmission-line system taught by Jaffe is that a biasing means is specifically required for proper operation. At very high frequencies, such biasing means would introduce objectionable perturbations in the output signal due to parasitic elements of such biasing means. Such perturbations could be minimized by careful design and implementation of the biasing means, but the system complexity would be objectionably increased. The nonlinear transmission line according to the present invention eliminates that limitation in the linear transmission-line system taught by Jaffe by novel configuration and operation which eliminates the requirement for a separate biasing means. The elimination of a separate biasing means in a nonlinear transmission line is a significant improvement over the prior art provided by the nonlinear transmission line according to the present invention.
Still another limitation in the linear transmission-line system taught by Jaffe is that such system is specifically required to be of a grounded microstrip line construction as referenced hereinabove. Such a line configuration severely limits the optimization and application of such a linear transmission-line system as taught by Jaffe. The nonlinear transmission line according to the present invention overcomes that limitation in a nonlinear transmission line by providing for various different basic transmission-line configurations to provide optimum performance needed in applications that will become apparent by application of the invention. For example, the nonlinear transmission line according to the present invention may be configured as coplanar transmission line to provide improved performance at extremely high frequencies and simplified construction by planar processes, or the transmission line according to the present invention may be configured as a simple parallel-plate transmission line for use in balanced-signal applications. The capability to provide different basic transmission-line structures in a nonlinear transmission line is a significant improvement over the prior art provided by the nonlinear transmission line according to the present invention.
Another integrated semiconductor device comprising a linear transmission-line structure is described in U.S. Pat. No. 4,229,717 entitled "VOLTAGE CONTROLLED SLOW WAVE TRANSMISSION LINE," to Krone, et. al. Krone teaches a substantially standard microstrip transmission-line structure as is common in the art. Such a linear transmission line as taught by Krone comprises a semiconductor slab on which a layer of insulating dielectric material is deposited on one surface and with a narrow strip conductor further deposited on the insulating dielectric layer. The insulating dielectric material as taught by Krone exhibits substantially no nonlinear effect. A grounded conductive plane is deposited on the other surface of the semiconductor slab. The strip conductor is substantially of a more narrow geometry than the ground plane such that the structure taught by Krone is substantially a microstrip transmission-line structure. The characteristics of the linear transmission line taught by Krone are controlled by application of an external control voltage. Further, the materials and specific construction of the linear transmission line taught by Krone limits operation to comparatively low frequencies in the slow-wave mode. Specifically, as previously described hereinabove with reference to the linear transmission line taught by Jaffe, the linear transmission line taught by Krone, in order to provide linear operation as taught by Krone, exhibits substantially no variation int eh transmission-line capacitance as a function of the propagating signal. The transmission-line capacitance is a function of only a direct-current bias in a similar manner as taught by Jaffe. Such a line will operate as a conventional linear transmission line as taught by Krone and will not provide signal compression common in a nonlinear transmission line.
The nonlinear transmission line according to the present invention is significantly different from the linear transmission line taught by Krone in both form and function. Specifically, the primary purpose in the nonlinear transmission line according to the present invention is to provide in a transmission line very nonlinear characteristics and operation at extremely high frequencies. Whereas the linear transmission line taught by Krone provides linear transmission-line performance and specifically in the slow-wave mode, the nonlinear, high-frequency performance of the nonlinear transmission line according to the present invention comprising a novel semiconductor device construction is significantly different and novel from the prior art taught by Krone.
A limitation in the linear transmission line taught by Krone in nonlinear operation is that the strip conductor is of constant width and the semiconductor structure of constant properties all along the length of the linear transmission-line structure thus providing a homogeneous transmission line. Such a homogeneous transmission-line is required for linear operation. As previously referenced hereinabove with reference to the linear transmission line taught by Jaffe, such a homogeneous transmission-line structure cannot provide optimization in high-frequency, nonlinear operation. The nonlinear transmission line according to the present invention overcomes that limitation in the linear transmission-line system taught by Krone in the propagation of signals of comparatively high frequency by a novel variable structure providing different transmission-line characteristics at different positions along the length of the transmission line of the nonlinear transmission line according to the present invention. Such structure allows for optimization of performance and critical parameters in the nonlinear transmission line according to the present invention. The capability to optimize desired performance and to optimize critical parameters along the line length in a nonlinear transmission line are significant improvements over the prior art provided by the nonlinear transmission line according to the present invention.
Another limitation in the linear transmission line taught by Krone in extremely high-frequency operation is the requirement for application of a separate control voltage. As previously referenced hereinabove, the application of such a separate control voltage will introduce objectionable signal degradation in high-frequency operation and will objectionably increase complexity. The nonlinear transmission line according to the present invention requires no separate control voltage. The elimination of a separate control voltage in a nonlinear transmission line is still another significant and novel improvement over the prior art provided by the nonlinear transmission line according to the present invention.
Still another limitation in the linear transmission line taught by Krone in extremely high-frequency operation is that materials and constructions are specifically selected to provide operation in a slow-wave mode. The nonlinear transmission line according to the present invention comprises specific materials such as extremely high-frequency Gallium-Arsenide materials and specific constructions such as high-frequency coplanar transmission-line structures to provide nonlinear performance at extremely high frequencies. The use of materials and constructions providing very high-frequency performance in a nonlinear transmission line is yet another significant and novel improvement over the prior art provided by the nonlinear transmission line according to the present invention.
The use of semiconductor diodes to provide a nonlinear capacitance in a lumped-element transmission-line structure is also known in the prior art and is described in, for instance, a paper "Hyperabrupt-Doped GaAs Nonlinear Transmission Line For Picosecond Shockwave Generation" by C. J. Madden, et. al., Applied Physics Letters, Volume 54, Number 11, Mar., 1989, page 1019. The nonlinear transmission line taught by Madden comprises several discrete semiconductor diodes interconnected by suitable electrodes to form a lumped-element transmission-line structure. Since the several individual semiconductor diodes exhibit variable capacitance characteristics, the lumped-element transmission line taught by Madden exhibits nonlinear transmission-line characteristics. Each individual discrete semiconductor diode in the nonlinear transmission line taught by Madden is substantially a simple lumped-element diode structure as is common in the art of semiconductor diodes. Therefore, each individual diode structure alone as taught by Madden exhibits neither transmission-line characteristics nor any manner of nonlinear transmission-line characteristics.
The nonlinear transmission line according to the present invention is significantly different from the nonlinear transmission line taught by Madden. The nonlinear transmission line according to the present invention comprises a single semiconductor device of a novel construction and operation further comprising a continuous nonlinear transmission-line structure. Whereas the nonlinear transmission line according to the present invention is a continuous transmission-line structure, it is significantly different from the prior art taught by Madden comprising a lumped-element transmission-line structure further comprising several individual semiconductor diodes where each such individual semiconductor diode is of a generally standard construction as is common in the prior art of semiconductor diodes.
A limitation in the lumped-element nonlinear transmission line taught by Madden is that it exhibits a characteristic cutoff frequency as is common in the art of lumped-element transmission lines. That cutoff frequency is determined by the capacitance value of each of the several semiconductor diodes and the value of the inductance of the interconnecting electrodes as is well known in the art of lumped-element transmission lines. In order to achieve high cutoff frequencies, the values of that capacitance and inductance must be made as small as possible. Correspondingly, in order to provide small values of capacitance and inductance, small feature size in the semiconductor structures and metallizations are required. Since such small feature sizes are limited by the physical limitations of the manufacturing process, the lumped-element nonlinear transmission line taught by Madden is then limited in maximum operating frequency by the physical limitations in the manufacturing process. The nonlinear transmission line according to the present invention overcomes that limitation in the lumped-element nonlinear transmission line taught by Madden by the use of a continuous transmission-line structure. Whereas the nonlinear transmission line according to the present invention is continuous in nature, it does not exhibit the objectionable limiting characteristic cutoff frequency common in the lumped-element nonlinear transmission lines of the prior art. The elimination of a characteristic cutoff frequency in a nonlinear transmission line is a significant improvement over the prior art provided by the nonlinear transmission line according to the present invention.
Another limitation in the lumped-element nonlinear transmission line taught by Madden is that only a small portion of the total electrical line length is active in producing the desired nonlinear effect. That results in excessive electrical loss. To explain in more detail, the length of electrode utilized to interconnect between adjacent individual diodes in the lumpedelement nonlinear transmission line taught by Madden is much longer electrically than the actual diode electrical dimension on the line. The interconnecting electrode length in the lumpedelement transmission line cannot be reduced in length since such length is selected to provide the specific inductance required for Therefore, the total line loss in the lumped-element nonlinear transmission line as taught by Madden is the sum of the losses contributed by the actual individual diode structures and the interconnecting electrodes. Whereas the total interconnecting electrode electrical length is very much longer than the total combined electrical length of the several individual diodes in the line taught by Madden, the total loss in the line taught by Madden is dominated by the interconnecting electrodes. Whereas such electrodes do not contribute to the desired nonlinear effect in such a line, any loss as a consequence of such electrodes is an undesirable excess loss. The nonlinear transmission line according to the present invention overcomes that limitation in the line taught by Madden by a novel continuous nonlinear transmission line which eliminates the need for such inter-diode interconnecting electrodes which do not contribute to the desired nonlinear effect. Whereas the nonlinear transmission line according to the present invention comprises only a single transmission-line element, the need for objectionable inter-diode interconnecting electrodes is totally eliminated. The elimination of the inter-diode interconnecting electrodes in a nonlinear transmission line is a significant improvement over the prior art provided by the nonlinear transmission line according to the present invention.
Another limitation in the lumped-element nonlinear transmission line as taught by Madden is that the extremely small feature sizes necessary to provide the small values of capacitance and inductance required to maximize the line cutoff frequency significantly reduces the maximum voltage capabilities of such a line. The dielectric strength of the various materials limits the maximum voltage that may be applied. As the feature size of the various devices and the device spacing are reduced, the maximum operating voltage is reduced accordingly. That limitation severely limits the maximum operating voltage. Therefore, the maximum amplitude of the output signal that may be delivered by the lumped-element nonlinear transmission line taught by Madden is substantially limited. The nonlinear transmission line according to the present invention overcomes that limitation of the prior art nonlinear transmission lines by a novel continuous nonlinear transmission-line structure comprising a single semiconductor device. Whereas the continuous nonlinear transmission line according to the present invention exhibits no characteristic cutoff frequency, device feature sizes are not directly a function of maximum desired operating frequency. Feature sizes may be selected to optimize other operating parameters, such as maximum output voltage for example. Whereas a nonlinear transmission line according to the present invention may be effected as a single semiconductor diode of novel configuration, operating voltages available in such a semiconductor diode embodiment of the nonlinear transmission line according to the present invention are substantially similar to operating voltages available in conventional semiconductor diodes common in the prior art. Operating voltages as high as 1000V or higher and correspondingly high output signals, either voltage or current, may be provided in a nonlinear transmission line according to the present invention. The capability of operation at high operating voltages and the capability of delivering output signals of high voltage or current in a nonlinear transmission line are significant improvements over the prior art provided by the nonlinear transmission line according to the present invention.
Still another limitation in the lumped-element nonlinear transmission line taught by Madden is that the very small feature sizes and the multiple elements required in such a lumped-element nonlinear transmission line serve to reduce the yield of acceptable devices available in manufacture. Such a reduction in yield increases the manufacturing cost of such devices. The nonlinear transmission line according to the present invention overcomes that limitation by reduction of device complexity and by permitting application of feature sizes consistent with high manufacturing yield. Since the nonlinear transmission line according to the present invention comprises only a single semiconductor element, its complexity is significantly less than that of the prior art taught by Madden. The reduction in complexity and the ability to select feature size to improve manufacturing yield without compromise in performance in a nonlinear transmission line are significant improvements over the prior art provided by the nonlinear transmission line according to the present invention.
A semiconductor-diode embodiment of the nonlinear transmission line according to the present invention is significantly different from semiconductor diodes common in the prior art in both structure and operation. Semiconductor diodes of the prior art typically comprise an anode and a cathode electrode where the length and width of an electrode are approximately similar. A signal is applied in a semiconductor diode of the prior art homogeneously at each electrode such that the instantaneous voltage at any position on an electrode is substantially equal to the voltage at any other position on that electrode at the same instant in time.
A semiconductor-diode embodiment of the nonlinear transmission line according to the present invention comprises a semiconductor-diode junction that is very long with-respect-to its width. Similarly, each diode electrode is very long and narrow. That structure is significantly different from typical semiconductor diodes of the prior art. Further, an input signal is applied to a semiconductor-diode embodiment of the present invention at one end of the diode structure between the two diode electrodes. That novel signal application configuration results in a traveling wave being launched between the two long, narrow diode electrodes such that the electrodes act as the conductors of a transmission line. Therefore, the instantaneous voltage at one position on an electrode of the nonlinear transmission line according to the present invention is not necessarily equal to the voltage at some other position on that electrode at the same instant in time.
Further, whereas a semiconductor diode structure comprising a nonlinear transmission line according to the present invention is comparatively long, the properties of such semiconductor diode may be made different at different positions along such a long semiconductor diode structure. The selection of such different properties at different positions along the length of the nonlinear transmission line according to the present invention allows optimization for specific operation, for example minimum loss or maximum nonlinear effect.
The configuration of a semiconductor diode comprising a long, narrow junction of different properties along the length of such semiconductor diode and the launching of a traveling-wave signal along the diode electrodes in a nonlinear transmission-line mode as provided in the nonlinear transmission line according to the present invention are novel in the art of semiconductor diodes.
An embodiment of the nonlinear transmission line according to the present invention comprising an insulating layer on a semiconductor substrate, an MOS capacitor for example, is significantly different from semiconductor capacitors common in the prior art in both structure and operation for the same reasons as identified hereinabove for a semiconductor-diode embodiment of the present invention.
The nonlinear transmission line according to the present invention comprising an improved nonlinear transmission line provides higher operating frequency and higher operating voltage over that available in nonlinear transmission lines of the prior art. Additionally, the nonlinear transmission line according to the present invention provides in a nonlinear transmission line operation over a very wide temperature range which is a very significant improvement over the prior art. Also, the wider latitude in selection of device geometry provided in the nonlinear transmission line according to the present invention providing for optimization of various operating parameters is another significant improvement over the prior art. Further, the reduced complexity provided in the nonlinear transmission line according to the present invention improves production yield and in turn reduces cost in a nonlinear transmission line over that provided by the prior art.